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Thermometers and Thermobarometers in Granitic Systems

Department of Earth Sciences, University of Southern California, Los Angeles, California, 90089-0740, U.S.A., anderson@usc.edu

Andrew P. Barth

Department of Earth Sciences, Indiana University-Purdue University, Indianapolis, Indiana, 46202, U.S.A.

Joseph L. Wooden and Frank Mazdab

U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California, 94205, U.S.A.

The first 20% of the full text of this article appears below.

	INTRODUCTION

 
Recent advances in our ability to resolve the P-T-t (pressure-temperature-time) paths of orogenic terranes have continued to re-emphasize the common marriage between petrology and tectonics. The importance of determining the P-T history of metamorphic rocks, as through chemical analysis of mineral zoning profiles and inclusions initially paved the way into this area of our science (e.g., Essene 1982, 1989; Hoisch 1991; Speer 1993). Parallel success has been demonstrated for igneous systems (see review by Anderson 1996), with the derived information being primarily related to ascent and/or cooling of the crystallizing magma. And, in multiply-intruded terranes, where the ages of intrusion span a significant portion of the orogenic deformation, dated plutons act as important crustal "nails," which offer first-order insight into the descent and/or ascent of deformed crust during the orogenic process.

Emplacement barometry can be retrieved from contact metamorphic rocks where the enclosing host assemblages contain appropriate compositions for barometric calculations. However, in pervasively intruded terranes where a range of ages of magmatic activity exists, multiple and extensive overprinting makes the use of host metamorphic mineral assemblages and mineral compositions challenging. In contrast, several igneous minerals are less prone to the loss of primary compositions, in part due to the inherent insensitivity of the rock to high-grade subsolidus reequilibration and the paucity of post-crystallization fluids. However, there are exceptions to this generalization and below we discuss an example where high-T, post-emplacement fluids have altered the entire margin of the Mt. Stuart batholith of Washington.

In general, many igneous rocks lack mineral assemblages suitable for thermobarometric analysis. Either the thermodynamic variance is high or, in specific cases, the phases are characterized by component exchanges easily reset during slow cooling or later thermal events. Two feldspar and iron-titanium oxide thermometry . . . [Full Text of this Article]

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